From Soft would definitely need to only pick certain elements from Souls/Bloodborne, and then add a lot of stuff that feels more Castlevania. I think the combat in Bloodborne isn't half bad for a CV, but I'd prefer a slightly different feel to it.
But I would 100% trust them with the visuals, the atmosphere and the level design/level progression. Maybe narrow the gameworld down a bit and make sliiiightly more linear, with less backtracking, but still way more room for exploration than in something like LoS.
I just wanna see From Softs take on classic CV bosses like Medusa, Death, Legion and Dracula.
As for the difficulty I'd say it would have to be changed, or at least different. Castlevania can be hard but I think it should be hard in a different way than Souls.